1. Field of the Invention
The present invention relates to a substitutional electroless gold plating solution and an electroless gold plating method. More particularly, the present invention relates to a substitutional electroless gold plating solution and an electroless gold plating method for applying electroless gold plating to a surface of nickel or a nickel alloy. In addition, the present invention relates to a semiconductor device provided with a gold plating layer formed using the gold plating solution of the present invention.
2. Description of the Related Art
Semiconductor devices, as is illustrated in FIG. 8 which is referred to hereinafter to explain the present invention, include a so-called xe2x80x9cBGA (Ball Grid Array)xe2x80x9d type semiconductor device 16, in which solder balls 14 as ball-like terminals for external connection terminals are disposed on ball-like terminal mounting pads 12 as terminal mounting pads for external connection, that are disposed on one of the surface sides of a circuit substrate.
In this BGA type semiconductor device 16, the ball-like terminal mounting pads 12 are formed at the end portion of conductor patterns 22 that are wire-bonded and electrically connected to a semiconductor chip 20.
To apply the solder ball 14 to each of the ball-like terminal mounting pads 12, as illustrated in the step (A) of FIG. 9, electroplating of nickel or electroless nickel plating is applied to the surface of the ball-like terminal mounting pad 12 made of copper and formed at the end portion of the conductor pattern 22, so as to form a nickel layer 24 made of nickel or a nickel alloy. Electroless gold plating is applied further to the surface of this nickel layer 24 to form a thin film-like gold plating layer 26 having a thickness of not greater than 0.1 xcexcm. This thin film-like gold plating 26 prevents surface oxidation of the nickel layer 24 and secures excellent solderability.
Next, as illustrated in the step (B) of FIG. 9, a solder ball is placed and caused to reflow at each predetermined position of the surface of thin film-like gold plating layer 26 in such a manner as to form a eutectic alloy between the solder and gold at the junction of the solder ball 14 and the gold plating 26. In this way, the solder ball 14 can be fixed to the nickel layer 24 formed on the surface of the ball-like terminal mounting pad 12.
In the semiconductor devices mentioned above, fixing strength (shear strength) between the solder ball 14 immediately after reflow and the nickel layer 24 formed on the surface of the ball-like terminal mounting pad 12 has a value sufficient for practical use.
However, it has been found in the prior art semiconductor devices that when heat history is applied to the solder ball 14 fixed to the ball-like terminal mounting pad 12, such as heat history that heats the solder ball 14 fixed to the ball-like terminal mounting pad 12 under a predetermined condition (at 220xc2x0 C. for 45 seconds) and then cools it immediately to a room temperature, the shear strength of the solder ball 14 drops drastically.
The drastic drop of shear strength of the solder ball 14 resulting from such a heat history can be prevented when a palladium plating layer is formed between the nickel layer 24 and the thin film-like gold plating layer 26. However, the formation of the palladium plating layer between the nickel layer 24 and the thin film-like gold plating layer 26 renders the production process complicated. Moreover, since palladium is an expensive metal, the production cost of the product becomes high in addition to complication of the production process.
Therefore, means has been desired that can minimize the drop of shear strength of the solder ball 14 fixed to the nickel layer resulting from the heat history even when the thin film-like gold plating layer 26 is directly formed on the surface of the nickel layer 24 without forming the palladium plating layer.
It is therefore an object of the present invention to provide means capable of minimizing the drop of shear strength of the solder ball due to heat history even when thin film-like gold plating layer is directly formed on the surface of nickel or a nickel alloy.
That is, the first object of the present invention is to provide a gold plating solution useful in the direct application of a gold plating layer onto a surface of nickel or a nickel alloy.
The second object of the present invention is to provide a gold plating method useful in the direct application of a gold plating layer onto a surface of nickel or a nickel alloy, without causing reduction of shear strength of the solder ball.
The third object of the present invention is to provide a semiconductor device provided with a gold plating layer as one constitutional member of the terminal mounting pad.
The above objects and other objects of the present invention will be appreciated from the following detailed description of the present invention.
The inventors of the present invention have conducted the following studies to examine the cause of the drop of shear strength of the solder ball fixed to the surface of nickel or the nickel alloy (hereinafter called simply the xe2x80x9cnickel surfacexe2x80x9d in some cases).
First, substitutional electroless gold plating was applied to the nickel surface to directly form the thin film-like gold plating layer, and the thin film-like gold plating layer was then peeled to inspect the condition of the nickel surface through an electron microscope. It has been found that the nickel surface is coarsened at the level at which shear strength of the solder ball drops greatly due to the heat history, but this surface coarseness of nickel after peeling of the thin film-like gold layer is relatively small at the level at which the drop of shear strength of the solder ball due to the heat history is small.
It has also been found that the degree of surface coarseness varies depending on the substitutional electroless gold plating solution used for forming the thin film-like gold plating layer by substitutional electroless gold plating.
Therefore, the present inventors have examined the substitutional electroless gold plating solutions that can relatively reduce the surface coarseness of nickel after the thin film-like gold plating layer is peeled. As a result, the present inventors have acquired the observation that the surface coarseness of nickel can be minimized by using a substitutional electroless gold plating solution that contains tetraethylenepentamine as a straight chain alkylamine, hypophosphite as a reducing agent of nickel or a nickel alloy, and a gold cyanide as a gold source, even when the thin film-like gold plating layer formed on the nickel surface by electroless gold plating is peeled. The present invention has been thus completed.
That is, the present invention resides in a substitutional electroless gold plating solution for applying electroless gold plating to a surface of nickel or a nickel alloy, and the substitutional electroless gold plating solution is characterized in that a straight chain alkylamine, a reducing agent of nickel or a nickel alloy, and gold cyanide as a gold source, are blended to the substitutional electroless gold plating solution.
Further, the present invention resides in a method of electroless gold plating to a surface of nickel or a nickel alloy by using the substitutional electroless gold plating solution of the present invention. That is, the electroless gold plating method is characterized in that a straight chain alklylamine, a reducing agent of nickel or a nickel alloy, and gold cyanide as a gold source are blended to the substitutional electroless gold plating solution.
In the present invention, it is preferred to use triethylenetetramine or tetraethylenepentamine as the straight chain alkylamine, and hypophosphite or a hydrazine compound as the reducing agent.
When the portion to which electroless gold plating is applied is the surface portion, that is made of nickel or a nickel alloy, of a conductor pattern formed on a circuit substrate for mounting electronic components, there can be obtained an electronic device in which solder balls as external connection terminals fixed to the conductor pattern have excellent heat resistance.
Furthermore, the present invention resides in a semiconductor device comprising a gold plating layer on a layer of nickel or a nickel alloy, said gold plating layer being formed from a substitutional electroless gold plating solution which comprises a straight chain alkylamine, nickel or a nickel alloy as a reducing agent, and a gold cyanide as a gold source.
When a thin film-like gold plating layer is formed on the surface made of nickel or a nickel alloy, that is, nickel surface, by electroless gold plating that uses the substitutional electroless gold plating solution according to the present invention and furthermore when heat history that heats the solder balls fixed to predetermined positions of this thin film-like gold plating layer by a reflow operation under a predetermined condition (at 220xc2x0 C. for 45 seconds, for example) and then cools them immediately to room temperature, is applied to the solder balls, the drop of fixing strength (shear strength) of the solder balls due to such a heat history is extremely small.
The reason why the drop of the fixing strength (shear strength) of the soldered members such as the solder balls resulting from the heat history is extremely small when the substitutional electroless gold plating solution is used has not yet been clarified completely, but may be assumed to be as follows.
The thin film-like gold plating layer formed on the nickel surface by using the substitutional electroless gold plating solution according to the present invention is peeled, and the nickel surface is inspected through an electron microscope. It is found that the nickel surface is under the condition substantially equivalent to the surface condition before electroless gold plating. It can be assumed therefore that the drop of fixing strength due to the heat history can be reduced extremely remarkably because the solder member such as the solder balls is a allowed to adhere to the nickel surface by reflow.